1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly to a method of driving a plasma display panel, which can improve display quality.
2. Description of the Background Art
In the recent information society, the importance of a display device as a visual information transmission medium has been stressed than ever. A cathode-ray tube or a Braun tube which is being widely used is heavy and bulky. Hence, various types of flat panel display devices which are capable of overcoming the disadvantages of the cathode-ray tube are being developed.
These flat panel display devices include a plasma display panel (PDP), a field emission display, an electro-luminescence, etc.
The PDP out of these devices displays images and moving pictures, including characters or graphics, by irradiating phosphors by ultraviolet rays of 147 nm generated during a discharge of a mixed gas of He+Xe, Ne+Xe or He+Xe+Ne. The PDP is not only easy to make its thickness thin and its size large but also feasible to greatly improve picture quality thanks to the recent development of technology.
Especially, a tri-electrode AC (Alternative Current) surface-discharge PDP lowers a voltage necessary for a discharge by accumulating barrier charges using a dielectric layer, and has the advantages of a low-voltage driving operation and long life.
FIG. 1 is a perspective view illustrating a discharge cell of a general tri-electrode AC surface-discharge PDP.
Referring to FIG. 1, a discharge cell of the PDP includes a scan electrode Y and a sustain electrode Z formed both on an upper substrate 10, and an address electrode X formed on a lower substrate 18. The scan and sustain electrodes Y and Z include transparent electrodes 12Y and 12Z, respectively, and includes metal bus electrodes 13Y and 13Z, respectively, which are narrower in line width than the transparent electrodes 12Y and 12Z and formed at the edges of the transparent electrodes 12Y and 12Z.
The transparent electrodes 12Y and 12Z formed on the upper substrate 10 are made of Indium-Tin-Oxide (ITO). The metal bus electrodes 13Y and 13Z formed on the transparent electrodes 12Y and 12Z are made of metal, such as chrome (Cr), serving to reduce a drop in a voltage caused by the transparent electrodes 12Y and 12Z having high resistance. An upper dielectric layer 14 and a protective layer 16 are formed on the upper substrate 10 on which the scan and sustain electrodes are formed in parallel. Barrier charges generated during a plasma discharge are formed on the upper dielectric layer 14. The protective layer 16 protects the upper dielectric layer 14 from damaging by a sputtering generated during the plasma discharge and also increases the efficiency of secondary electron emission. A magnesium oxide (MgO) is usually used as the protective layer 16.
A lower dielectric layer 22 and a barrier rib 24 are formed on the lower substrate 18 on which the address electrode X is formed. A phosphor layer 26 is coated over the surfaces of the lower dielectric layer 22 and the barrier rib 24. The address electrode X is perpendicular to the scan and sustain electrodes Y and Z. The barrier rib 24 is formed in parallel to the address electrode X and prevents ultraviolet rays and visual rays generated by a discharge from leaking to an adjacent cell. The phosphor layer 26 is excited by ultraviolet rays generated during a plasma discharge and generates any one visual ray among red, green and blue. An inert mixed gas is injected into a discharge space provided between the upper and lower substrates 10 and 18 and the barrier rib 24.
In order to achieve a gray scale of an image, the PDP is driven on a time-division basis by dividing one frame into subfields each having the different number of irradiations. Each subfield has a reset period for resetting the entire screen, an address period for selecting a scan line and selecting a cell in the selected scan line, and a sustain period for achieving a gray scale according to the number of discharges.
The reset period is further divided into a set-up period for supplying a ramp-up waveform and a set-down period for supplying a ramp-down waveform. For example, if it is desired to display an image by 256 gradations, one frame period corresponding to 1/60 seconds (16.67 ms) is divided into 8 subfields SF1 to SF9, as shown in FIG. 2. Each of the 8 subframes SF1 to SF8 is further divided into the reset period, the address period and the sustain period as described above. The reset and address periods in each subfield are identical with respect to the respective subfields, while the sustain period increases at the rate of 2n (where n is 0, 1, 2, 3, 4, 5, 6 and 7).
FIG. 3 is a waveform diagram illustrating a driving method of the PDP.
Referring to FIG. 3, the PDP is separately driven according to a reset period for resetting the entire screen, an address period for selecting a cell, and a sustain period for sustaining a discharge of the selected cell.
During the reset period, a ramp-up waveform is simultaneously applied to all the scan electrodes Y during a set-up period. In this case, the scan electrodes Y are raised up to a voltage Vp for discharging cells. By this ramp-up waveform, a weak discharge occurs within the cells of the entire screen and wall charges are created within the cells. During a set-down period after the ramp-up waveform is supplied, a ramp-down waveform falling from a positive-polarity voltage lower than the peak voltage of the ramp-up waveform is simultaneously applied to all the scan electrodes Y. The ramp-down waveform creates a weak erase discharge within the cells, erasing unnecessary charges out of the wall charges and space charges generated by the set-up discharge and uniformly sustaining the wall charges necessary for an address discharge within the cells of the entire screen.
During the address period, a negative-polarity scan pulse Scan is sequentially applied to the scan electrodes Y and at the same time a positive-polarity data pulse is applied to the address electrodes. A difference between a voltage Vy of the scan pulse and a voltage Va of the data pulse is added to a wall voltage generated during the reset period to create the address discharge within the cells. The wall charges are generated within the selected cells by the address discharge.
Meanwhile, during the set-down period and the address period, a positive-polarity direct current (DC) voltage of a sustain voltage level Vs is supplied to the sustain electrodes Z.
During the sustain period, a sustain pulse Sus of the sustain voltage level Vs is alternatively applied to the scan electrodes Y and the sustain electrodes Z. Then the wall voltage within the cell is added to the sustain pulse Sus by the address discharge, and a sustain discharge is created between the scan and sustain electrodes Y and Z as a form of a surface discharge whenever each sustain pulse Sus is applied. Finally, after the sustain discharge is completed, an erase ramp waveform having a narrow pulse width is supplied to the sustain electrodes Z, erasing the wall charges within the cell.
In the above-described PDP, a preliminary waveform is supplied during a preliminary period as shown in FIG. 4 in order to ensure time to raise a plurality of voltage sources Vp, Vs, Vy and Va to desired voltages when a power source is turned on.
In this case, the data pulse is not supplied during the address period so as not to create a discharge during the preliminary period. During the reset and sustain periods, the same waveforms as FIG. 3 are applied and thus a detailed description thereof will not be given.
During the preliminary period, however, an undesired sustain discharge occurs, and an afterimage is displayed on the panel. In more detail, the power source of the PDP is randomly turned off by a user at any time, the wall charges remain within the discharge cells. Especially, such wall charges remain largely within the discharge cells displaying bright luminescence before the power source of the PDP is turned off and the undesired sustain discharge occurs during the sustain period of the preliminary period by these remaining charges. Consequently, the afterimage is displayed in a part of the discharge cells which have displayed a bright screen when the power source of the PDP is turned off.